Ever since it was first discovered by Sir Alexander Ogston in 1880, Staphylococcus aureus has been regarded as a serious threat to human health, capable of causing a multitude of infections. The rise of antibiotic-resistant strains in the 1960s and 1970s, particularly methicillin-resistant S. aureus (MRSA), has created additional therapeutic challenges. Currently, MRSA strains account for >50% of all S. aureus isolates causing clinical disease in the US. This is a much higher percentage compared to other countries, such as France at 14.5% and the Netherlands at 3.1%. In a review of 31 observational studies from Western Europe, the authors found that the percentage of MRSA among S. aureus clinical isolates ranged between 5% and 54%, but was limited by the different methodologies used in the studies.
Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium responsible for several difficult-to-treat infections in humans. It is also called multidrug-resistant Staphylococcus aureus and oxacillin-resistant Staphylococcus aureus (ORSA). MRSA is any strain of Staphylococcus aureus that has developed resistance to beta-lactam antibiotics, which include the penicillins (such as methicillin, dicloxam, nafcillin and oxacillin) and the cephalosporins. Strains unable to resist these antibiotics are classified as methicillin-sensitive Staphylococcus aureus (MSSA). The development of such resistance does not cause the organism to be more intrinsically virulent than strains of Staphylococcus aureus that have no antibiotic resistance, but resistance does make MRSA infection more difficult to treat with standard types of antibiotics and thus more dangerous.
MRSA is especially troublesome in hospitals, prisons, schools, and nursing homes, where patients with open wounds, invasive devices, and weakened immune systems are at greater risk of infection than the general public.
MRSA strains are prevalent bacterial pathogens that cause both health care- and community-associated infections. Increasing resistance to commonly prescribed antibiotics has made MRSA a serious threat to public health throughout the world. The USA300 strain of MRSA has been responsible for an epidemic of community-associated infections in the US, mostly involving skin and soft tissue but also more serious invasive syndromes such as pneumonia, severe sepsis and endocarditis. MRSA strains are particularly serious and potentially lethal pathogens that possess virulence mechanisms including toxins, adhesins, enzymes and immunomodulators. One of these is Panton-Valentine leukocidin (PVL), a toxin associated with abscess formation and severe necrotizing pneumonia.
Initially, MRSA strains afflicted hospitalized patients and those with chronic illnesses. The 1990s saw the emergence of community-associated MRSA (CA-MRSA) strains that primarily caused skin and soft tissue infections (SSTIs) in otherwise healthy individuals, often children. These strains quickly led to an epidemic of CA-MRSA infections including some with severe consequences, for example, community-acquired pneumonia with high mortality rates. The high prevalence of CA-MRSA among infecting MRSA strains in the US is mostly due to the Panton-Valentine leukocidin (PVL)-positive USA300 clone, while in Europe the predominant strain of CA-MRSA is a PVL-positive ST80 clone. A mathematical model predicted that CA-MRSA will become the dominant MRSA strain in hospitals because of the expanding community reservoir, CA-MRSA strains are more fit (higher replicative capacity) than hospital-associated types and that CA-MRSA infections will become increasingly severe (D'Agata et al., Clin. Infect. Dis. 48, 274-284, 2009).
Agents directed against the virulence mechanisms of MRSA strains would have several advantages compared to antibiotics. First, there would be no selective pressure exerted on other nonpathogenic, commensal bacteria. Second, the associated toxicities of antibiotics (e.g. allergic reactions, nephrotoxicity and Clostridium difficile infection) may be avoided. Third, limiting antibiotics may decrease the development of drug-resistant bacteria. Combining antivirulence therapies with traditional antibiotics has the potential to change the paradigm of how MRSA infections are managed. Since bacterial survival is not impacted by the function of its virulence mechanisms, it is possible that resistance to antivirulence therapy would be slow to develop. One potential strategy is to inhibit the agr operon. In vitro experiments have shown that variants of autoinducing peptide (AIP) inhibit AgrC function. An in vivo study demonstrated that administering AIP-2 concurrently with an agr type 1 strain reduced abscess formation (Wright et al., Proc. Natl. Acad. Sci. USA 102, 1691-1696, 2005). However, agr inhibitors can promote biofilm formation, which could result in chronic S. aureus infections (Beenken et al., PLoS ONE 5, e10790, 2010). Hence, further investigation on this approach is needed.
Another strategy for devices is the use of nanomaterials, defined as materials with at least one dimension less than 100 nm, to prevent the formation of biofilms (Taylor & Webster, Int. J. Nanomedicine 6, 1463-1473, 2011). SilverPage lined urinary catheters and central venous catheters are used in clinical practice to lower the risk of health care-associated infections (Raad et al., Antimicrob. Agents Chemother. 56, 935-941, 2012). Decreasing the particle size of silver down to the nanometer range increases the surface area, which improves the antibacterial activity of the material (Taylor & Webster, Int. J. Nanomedicine 6, 1463-1473, 2011). Staphyloxanthin is a pigment of S. aureus that helps it resist reactive oxygen species such as those released by neutrophils. Early steps in staphyloxanthin production are similar to those in cholesterol production. A human squalene synthase inhibitor blocked staphyloxanthin biosynthesis in vitro, resulting in nonpigmented bacteria that were more susceptible to killing by human blood and clearance by the innate immune system in a mouse model (Liu et al., Science 319, 1391-1394, 2008). Statins were shown to enhance S. aureus clearance by phagocytes through production of antibacterial DNA-based extracellular traps by human and murine neutrophils, macrophages and monocytes (Chow et al., Cell Host Microbe 8, 445-454, 2010).
For CA-MRSA infections, one specific target is PVL toxin, and antibody against it is under investigation as a potential vaccine. However, in a study on antibody levels against PVL in children with PVL-positive MRSA infections, neutralizing antibody against PVL was not protective against primary or recurrent CA-MRSA skin infections (Hermos et al., Clin. Infect. Dis. 51, 1138-1146, 2010). Other investigators, using a murine model of dermonecrosis, evaluated an agonist of human C5a called EP67 for its ability to induce host immunity against CA-MRSA (Sheen et al., Vaccine 30, 9-13, 2011). EP67 was effective in limiting the infection through the promotion of cytokine synthesis and neutrophil influx. This promising finding may warrant further investigation in humans.
Peptidoglycan (PG) comprises approximately 50% of the cell wall of S. aureus. A PG-based vaccine against S. aureus, A170PG, was shown to be protective in a mouse model against several strains of MRSA including A174, A175, A176 and RIMD31092 (Capparelli et al., PLoS ONE 6, e28377, 2011). The protection correlated with increased survival and reduced colonization and lasted at least 40 weeks. One caveat with this study is that the mouse strain used does not closely mimic human infection because mice do not have pre-existing antibodies to S. aureus. In June 2011, Merck and Intercell announced the termination of phase II/III development of V170, a subunit vaccine containing the S. aureus antigen IsdB, which is a cell surface localized iron-regulated protein (Etz et al., Proc. Natl. Acad. Sci. USA 99, 6573-6578, 2002). Safety concerns were cited due to an increase in overall mortality and multi-organ dysfunction in the vaccine recipients compared to those who received placebo.